UASB (upflow anaerobic sludge blanket) methods are high-load and high-speed treatments utilized to anaerobically treat organic wastewater by allowing the organic wastewater containing soluble BOD to flow upward in a reaction tank and to be brought into contact with a sludge blanket formed by a dense and highly settling granule sludge in the tank. According to such a method, indigestive solid organic substances are separated before individual treatments, and rapidly digestible soluble organic substances alone are anaerobically treated with high load and high speed by using granule sludge having a high concentration of anaerobic microorganisms. An advanced form of UASB method is EGSB (expanded granule sludge blanket) method that enables higher loads of anaerobic treatment by allowing wastewater to flow at a faster rate in a taller reaction tank so as to expand a sludge blanket.
In granule sludge anaerobic treatments such as UASB method and EGSB method, sludge containing anaerobic microorganisms forms granules inside reactor. Because such anaerobic treatment methods achieve higher sludge concentration compared to fixed-bed or fluidized-bed treatments in which sludge is supported on carriers, high-load operation is feasible. Further, quick and efficient startup is possible simply by acquiring extra sludge from other treatment systems in operation.
These methods using granule sludge are very efficient when wastewater has high COD concentration (generally CODCr concentration of not less than 2000 mg/L). When the COD concentration is low (generally CODCr concentration of not more than 2000 mg/L), however, a large amount of water has to be flowed in a reaction tank, causing granules to outflow. Consequently, stable operation may be infeasible.
In the case where the types or compositions of wastewater to be treated by these methods are such that the formation of granules becomes difficult, the granules initially added are gradually broken or washed out and can possibly render the system inoperable.
In contrast, methods utilizing non-biological fluidized carriers are advantageously applicable even to wastewater having low COD concentration or wastewater accompanying granule breakage because outflow of the carriers from reaction tanks can be prevented by mechanical apparatus such as screens. Fluidized carriers provide a place for the growth of microorganisms.
Organic wastewater containing polymer components such as sugars and proteins are treated with a two-phase anaerobic treatment apparatus utilizing an acidification tank and an anaerobic reaction tank. Such organic wastewater is introduced into the acidification tank where polymers in the wastewater are decomposed into low-molecular organic acids such as acetic acid, propionic acid, and thereafter the waters is treated in the reaction tank containing granules or carriers.
When wastewater contains only compounds directly digestible by methanogenic bacteria such as methanol, or acetic acid, efficient treatment is possible with a single-phase treatment apparatus without an acidification tank by allowing the contaminated waters to flow directly through a reaction tank containing granules or carriers. Contaminated water containing large amounts of polymer components may be introduced into an acidification tank beforehand in order to decompose the polymer components into low-molecular compounds, whereby such wastewater can be treated in a subsequent reaction tank with high treatment efficiency.
Patent Document 1 describes an anaerobic treatment method in which an organic wastewater is treated in an acidification tank and is thereafter treated in a UASB methane production tank while flowing upward through the methane production tank. In the anaerobic treatment described in Patent Document 1, the sugar/CODor ratio in the liquid to be introduced into the methane production tank is controlled such that granule sludge having high activity and high settling tendency in the UASB methane production tank.
Patent Document 2 describes a method for treating beer wastewater in which a beer wastewater with high concentration is treated in an acidification tank (a raw water control tank), thereafter diluted to a CODCr concentration of not more than 3000 mg/L, and introduced into a fluidized-bed methane fermentation tank containing carriers, thereby coping with variations in amounts and qualities of wastewater.